My 3 month old screams and fights all daytime naps. Have tried everything. What can I do?

Have patience. You are probably exhausted and need a break. Make sure the room in which he's napping is dark and quiet. If there are sounds coming from elsewhere, you can block that noise out either with soft music or white noise. White noise includes sounds like heart beats, static, ocean sounds, and other sounds that are meaningless but help block more distracting noises. Make sure he's fed, dry, etc..
Be consistent. Make sure that you have a consistent routine everyday - put him down at the same time and in the same place. Ensure that the daytime environment is not too noisy or busy - he should have a quiet place to sleep so he can settle. A white noise machine can help, as can a lullaby cd.